1. Field of the Invention
The present invention relates to an optical fiber suitable for long-haul transmission of optical signals having wavelengths different from each other, and an optical transmission line including the same.
2. Related Background Art
In an optical communication system using an optical fiber network, long-haul and large capacity optical communication is possible. Particularly, in the recent increase in capacity, a wavelength division multiplexing (WDM) technique which enables transmission of a plurality of optical signals having wavelengths different from each other is used. This optical communication system is constituted by an optical transmitter for outputting optical signals, an optical amplifier for amplifying the optical signals, an optical fiber as an optical transmission line for transmitting the optical signals, an optical receiver for receiving the optical signals, and the like.
Among these structural elements, in the optical amplifier which is indispensable for obtaining a high S/N ratio, a wavelength band (amplification wavelength band) in which optical signals can be amplified is conventionally 1530 to 1565 nm. Thus, the other elements constituting the optical communication system have been designed so that they operate excellently in this amplification wavelength band. For example, an optical transmission line disclosed in D. W. Peckham, et al., xe2x80x9cReduced Dispersion Slope, Non-Zero Dispersion Fiberxe2x80x9d, ECOCxe2x80x2 98, pp. 130-140, 1998 (first document) or U.S. Pat. No. 5,684,909 (second document) is designed so that a deviation of dispersion in this amplification wavelength band, that is, a dispersion slope becomes small.
The present inventors examined the conventional optical communication system having the above structure, and consequently, found the problems as follows:
That is, as the performance of the optical amplifier is improved, the amplification wavelength band of the optical amplifier is being expanded from the foregoing wavelength band (1530 to 1565 nm) to the wavelength band of 1530 to 1620 nm including a longer wavelength side. This fact is introduced by, for example, M. Kakui, et al., xe2x80x9cOptical Amplifications Characteristics around 1.58 xcexcm of Silica-Based Erbium-Doped Fibers Containing Phosphorous/Alumina as Codopantsxe2x80x9d, OAA""98, TuC3, pp. 107-110, 1998 (third document). As the amplification wavelength band of the optical amplifier is expanded, it is necessary that other elements are also designed so that they operate excellently in the expanded wavelength band of 1530 to 1620 nm. However, it was impossible to say that in the conventional optical fiber and the optical transmission line including the same, the dispersion slope is sufficiently small in the expanded amplification wavelength band of 1530 to 1620 nm.
For example, let us consider an optical transmission line in which a first optical fiber having a positive dispersion and a positive dispersion slope in the wavelength band of 1530 to 1620 nm and a second optical fiber having a negative dispersion and a positive dispersion slope in the wavelength band of 1530 to 1620 nm are connected to each other at a suitable length ratio. Incidentally, in this optical transmission line, it is assumed that a dispersion in the center wavelength 1575 nm of the wavelength band of 1530 to 1620 nm is 0, and a difference between the maximum value and the minimum value of the dispersion in the wavelength band of 1530 to 1620 nm is xcex94D. FIG. 1 is a graph showing the dispersion of each of the optical transmission line, the first optical fiber, and the second optical fiber, and in FIG. 1, a graph G110 indicates the dispersion of the first optical fiber, a graph G120 indicates the dispersion of the second optical fiber, and a graph G130 indicates the dispersion (obtained by the fiber length and dispersion value of each of the first and second optical fibers) of the optical transmission line including the first and second optical fibers.
FIG. 2 is a graph showing the relation between transmission distance and accumulated dispersion with respect to the optical transmission line having the foregoing structure. In FIG. 2, G210 indicates the relation between the transmission distance and the accumulated dispersion value in the case where the difference xcex94D is 3.6 ps/nm/km, G220 indicates the relation in the case where the difference xcex94D is 2.0 ps/nm/km, and G230 indicates the relation in the case where the difference xcex94D is 1.0 ps/nm/km. Besides, FIG. 2 shows a value (arrow A in the drawing) of the accumulated dispersion which becomes a transmission limit when the bit rate of optical signals is 10 Gb/s, and a value (arrow B in the drawing) of the accumulated dispersion which becomes a transmission limit when the bit rate of optical signals is 20 Gb/s.
In the optical transmission line disclosed in the first document, the dispersion slope in the wavelength band of 1530 to 1620 nm is 0.04 ps/nm2/km, and the difference xcex94D between the maximum value and the minimum value of the dispersion in this wavelength band is 3.6 ps/nm/km. Thus, in the case of the optical transmission line of the first document, as is understood from FIG. 2, the optical signals of a bit rate of 10 Gb/s can be transmitted only over a distance of about 550 km, and the optical signals of a bit rate of 20 Gb/s can be transmitted only over a distance of about 150 km. For reference, when the difference xcex94D is 2.0 ps/nm/km, the optical signals of a bit rate of 10 Gb/s can be transmitted over a distance of about 1000 km, and the optical signals of a bit rate of 20 Gb/s can be transmitted over a distance of about 250 km. Further, when the difference xcex94D is 1.0 ps/nm/km, the optical signals of a bit rate of 10 Gb/s can be transmitted over a distance of about 2000 km, and the optical signals of a bit rate of 20 Gb/s can be transmitted over a distance of about 500 km.
The present invention has been made to solve the foregoing problems, and has an object to provide an optical fiber having a structure suitable for long-haul transmission of a plurality of optical signals having wavelengths different from each other in a wavelength band of 1530 to 1620 nm, and an optical transmission line including the same.
An optical transmission line of the present invention is an optical fiber transmission line disposed in at least one of places between an optical transmitter and an optical receiver, between an optical transmitter and an optical repeater including an optical amplifier, between optical repeaters, and between an optical repeater and an optical receiving station.
The optical transmission line of the present invention includes one or more first optical fibers, and one or more second optical fibers. However, the optical transmission line may be constituted by one first optical fiber and one second optical fiber, and in the case where a plurality of first optical fibers and a plurality of second optical fibers are mutually fused and connected, the order of connection of these optical fibers may be arbitrary.
Each of the first optical fibers has a dispersion of +1.0 to +8.0 ps/nm/km in a wavelength band of 1530 to 1620 nm, and a difference between the maximum value and the minimum value of the dispersion is 3.0 ps/nm/km or less, preferably 2.0 ps/nm/km or less. Besides, each of the second optical fibers has a dispersion of xe2x88x921.0 to xe2x88x928.0 ps/nm/km in the above wavelength band, and a difference between the maximum value and the minimum value of the dispersion is 3.0 ps/nm/km or less, preferably 2.0 ps/nm/km or less. The optical transmission line is characterized in that in the above wavelength band, an average dispersion value obtained from each fiber length and each dispersion value of the first and second optical fibers is 2.0 ps/nm/km or less, preferably 1.0 ps/nm/km or less, more preferably 0.5 ps/nm/km or less.
Incidentally, in the above structure, it is preferable that the dispersion (average dispersion value) of the whole optical transmission line of the present invention has opposite signs at the wavelength of 1530 nm and the wavelength of 1620 nm. Besides; in the case where the dispersion of the whole optical transmission line of the present invention becomes 0 at any one (ideally, near the center wavelength of the wavelength band) of the wavelength band, it is preferable that the absolute value of the average dispersion value is 1.0 ps/nm/km or less, and preferably 0.5 ps/nm/km or less.
According to the optical transmission line having the structure as described above, in the wavelength band of 1530 to 1620 nm, when the difference xcex94D between the maximum value and the minimum value of the average dispersion value in the whole transmission line is 2.0 ps/nm/km or less (in the case where the average dispersion value becomes 0 at any one of the wavelength band, when the absolute value of the average dispersion value is 1.0 ps/nm/km or less), the optical signals of a bit rate of 10 Gb/s can be transmitted over a distance of about 1000 km, and the optical signals of a bit rate of 20 Gb/s can be transmitted over a distance of about 250 km. Further, in the wavelength band of 1530 to 1620 nm, when the difference xcex94D between the maximum value and the minimum value of the average dispersion value is 1.0 ps/nm/km or less (in the case where the average dispersion value becomes 0 at any one of the wavelength band, when the absolute value of the average dispersion value is 0.5 ps/nm/km or less), the optical signals of a bit rate of 10 Gb/s can be transmitted over a distance of about 2000 km, and the optical signals of a bit rate of 20 Gb/s can be transmitted over a distance of about 500 km.
Besides, it is preferable that each of the first and second optical fibers has an effective area of 40 xcexcm2 or more at a wavelength of 1550 nm. In this case, since the light intensity per unit sectional area becomes low, the occurrence of a nonlinear optical phenomenon such as four-wave mixing is suppressed. Thus, it is possible to increase the power of an optical signal transmitting through the optical transmission line, and it becomes possible to extend a transmission distance.
Incidentally, the effective area Aeff can be given by the following expression (1) as indicated in Japanese Patent Unexamined Publication No. Hei. 8-248251 (EP 0 724 171 A2).                               A          eff                =                  2          ⁢          π          ⁢                      xe2x80x83                    ⁢                                                    (                                                      ∫                    0                    ∞                                    ⁢                                                            E                      2                                        ⁢                    r                    ⁢                                          ⅆ                      r                                                                      )                            2                        /                          (                                                ∫                  0                  ∞                                ⁢                                                      E                    4                                    ⁢                  r                  ⁢                                      ⅆ                    r                                                              )                                                          (        1        )            
Where, E is an electric field of transmission light, and r is a distance from the center of a core in the direction of a radius.
Besides, it is preferable that each of the first and second optical fibers has a bending loss of 0.5 dB or less at a wavelength of 1620 nm when it is wound one turn at a diameter of 32 mm. By this, in the wavelength band of 1530 to 1620 nm, the bending loss can be made sufficiently low, and an increase in transmission loss due to formation of a cable or the like can be effectively suppressed.
An aspect of an optical fiber which can be applied to the optical transmission line having the structure as described above may be an optical fiber in which in the wavelength band of 1530 to 1620 nm, an absolute value of its dispersion is 1.0 to 8.0 ps/nm/km and a difference between a maximum value and a minimum value of the dispersion is 3.0 ps/nm/km or less, preferably 2.0 ps/nm/km or less, and its bending loss is 0.5 dB or less at a wavelength of 1620 nm when it is wound one turn at a diameter of 32 mm. Incidentally, it is preferable that the optical fiber of this aspect also has an effective area of 40 xcexcm2 or more at a wavelength of 1550 nm. Besides, in order to suppress an increase in bending loss, another aspect may be an optical fiber in which in the wavelength band of 1530 to 1620 nm, an absolute value of its dispersion is 1.0 to 8.0 ps/nm/km and a difference between a maximum value and a minimum value of the dispersion is 3.0 ps/nm/km or less, preferably 2.0 ps/nm/km or less, and its effective area is less than 60 xcexcm2 at a wavelength of 1550 nm. However, this optical fiber of the other aspect has an effective area of 40 xcexcm2 or more at the wavelength of 1550 nm.
The optical fiber of the respective aspects has a structure in which a first core, a second core, a third core, an inner cladding, and an outer cladding are sequentially provided, while an optical axis is made the center. The first core extends along a predetermined axis. The second core is provided so as to surround the first core and has a refractive index lower than that of the first core. The third core is provided so as to surround the second core and has a refractive index higher than that of the second core. The inner cladding is provided so as to surround the third core and has a refractive index lower than that of the third core. The outer cladding is provided so as to surround the inner cladding and has a refractive index higher than that of the inner cladding.
Further, as an optical fiber which can be applied to the optical transmission line of the present invention, a unitary optical fiber with no connection point including first and second portions having core diameters different from each other by 2% or more can be applied. Incidentally, this optical fiber is characterized in that the first portion has a dispersion of +1.0 to +8.0 ps/nm/km in the wavelength band of 1530 to 1620 nm, and a difference between a maximum value and a minimum value of the dispersion is 3.0 ps/nm/km or less, preferably 2.0 ps/nm/km or less, and the second portion includes a dispersion of xe2x88x921.0 to xe2x88x928.0 ps/nm/km in the wavelength band, and a difference between a maximum value and a minimum value of the dispersion is 3.0 ps/nm/km or less, preferably 2.0 ps/nm/km or less. The optical fiber in which the core diameter is changed along the longitudinal direction like this is obtained by changing wire drawing tension at the time of manufacture, by changing an outer diameter of a core portion in an optical fiber parent material along the longitudinal direction, or the like. Since this optical fiber is a unitary optical fiber (dispersion-managed optical fiber) subjected to dispersion-managed, when a plurality of dispersion-managed optical fibers are fused and connected to each other and an optical transmission line is constituted, it is not necessary to consider the dispersion of each of the optical fibers, and it is possible to easily constitute an optical transmission line which enables long-haul transmission of optical signals having a plurality of wavelengths in the wavelength band of 1530 to 1620 nm. Incidentally, it is preferable that this dispersion-managed optical fiber also has the structure as described above.
Even in the optical transmission line constituted by one dispersion-managed optical fiber as described above or in the optical transmission line in which the plurality of dispersion-managed optical fibers are fused and connected, it is characterized in that a difference between a maximum value and a minimum value of an average dispersion value in a wavelength band of 1530 to 1620 nm is 2.0 ps/nm/km or less, preferably 1.0 ps/nm/km or less. Even in this optical transmission line, in the wavelength band of 1530 to 1620 nm, the difference xcex94D between the maximum value and the minimum value of the average dispersion is 2.0 ps/nm/km or less, optical signals of a bit rate of 10 Gb/s can be transmitted over a distance of about 1000 km, and optical signals of a bit rate of 20 Gb/s can be transmitted over a distance of about 250 km. Further, in the wavelength band of 1530 to 1620 nm, when the difference xcex94D between the maximum value and the minimum value of the average dispersion value is 1.0 ps/nm/km or less, optical signals of a bit rate of 10 Gb/s can be transmitted over a distance of about 2000 km, and optical signals of a bit rate of 20 Gb/s can be transmitted over a distance of about 500 km.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.